Calculation methods and data storage
ATLAS provides everyday, finance, health and math tools. This document describes the implemented formulas and the limits of their results.
Content updated:
Formulas and assumptions
Everyday
- Discount calculator
- Final price = original price × (1 − discount rate ÷ 100)
- Split a bill
- Per person = bill × (1 + tip ÷ 100) ÷ people
Rounded up to the smallest currency unit. The last person can adjust for the rounding difference.
- Date difference
- End date − start date (excluding the start date, calendar days)
Finance
- Loan payment
- Amortized payment = P × r ÷ (1 − (1 + r)⁻ⁿ)
r is the monthly rate; n is the number of months. Fixed rate, end-of-month payments. Excludes fees, taxes and insurance.
- Savings
- Simple monthly savings interest = deposit × annual rate ÷ 100 ÷ 12 × n(n + 1) ÷ 2
Before tax, simple interest. Monthly deposits occur at the start of each month; lump-sum is deposited once. Actual bank day-count conventions may differ.
- Compound interest
- Future value = principal × (1 + annual rate ÷ 100 ÷ frequency)^(frequency × years)
Assumes a constant rate with no additional deposits, taxes, fees or inflation.
Health
- BMI
- BMI = weight (kg) ÷ height (m)²
An adult screening measure. Displays a numeric BMI without country-specific diagnosis; not a substitute for medical assessment.
CDC · BMI - Basal metabolic rate
- Mifflin–St Jeor: 10W + 6.25H − 5A + s (male +5, female −161)
W=kg, H=cm, A=age. Estimates adult resting energy expenditure; not for pregnancy, breastfeeding or growing children.
Mifflin et al., 1990 - Daily calorie needs
- Daily energy = Mifflin–St Jeor resting energy × activity multiplier
The default 1.2 multiplier is an illustrative low-activity assumption. Adjust for your needs. Adult estimate, not a prescribed diet.
Mifflin et al., 1990
Math
- Basic calculator
- Order: parentheses, multiplication/division, then addition/subtraction.
- Percentage
- Ratio = A ÷ B × 100 · change = (B − A) ÷ |A| × 100
- Unit converter
- Uses defined conversion factors: 1 in = 2.54 cm, 1 lb = 0.45359237 kg
Rounding and precision
Internal calculations use JavaScript double-precision floating point. Display rounds KRW to whole won, USD to two decimals, BMI to one decimal and kcal to whole numbers; other values show up to six decimals. Multiplying rounded display amounts may differ from internal totals.
Bill splitting rounds upward to the smallest currency unit and displays any surplus. Date differences use UTC calendar days and exclude the start day.
References
- CDC · Body Mass Index
- Mifflin et al. (1990) · Resting energy expenditure
- NIST · SI conversion factors
- Investor.gov · Compound interest
References support definitions and methods. ATLAS is not affiliated with these institutions and does not provide medical diagnosis or investment advice.
Country and language
Korean and English have separate URLs. The entry route uses an estimated country and saved language preference; explicit language URLs remain accessible. Country preferences affect currency and input units, not tax law or exchange rates. Switching country converts body measurements while keeping monetary numbers.
Inputs, saved history and shared links
Calculation happens in the browser. Favorites and the last 20 calculations are stored locally without account synchronization. History can be deleted individually or entirely, and clearing browser data removes it. Language and country preferences are saved in cookies valid for one year.
Shared links encode selected calculator inputs and country after # in the URL. This fragment is not sent in ordinary HTTP requests, but recipients and browser extensions can read it. The share button copies a link; it does not automatically send it to anyone.